1. Field of the Invention
The invention relates to an installation for the cataphoretic dip coating of articles, in particular vehicle bodies, comprising    a) a dip tank, which is partially filled with a coating liquid, into which the articles can be immersed;    b) a coating voltage source, to the negative terminal of which the immersed articles can be connected and the positive terminal of which is connected with at least one anode immersed in the coating liquid present in the dip tank;    c) an electrodialysis apparatus, which is arranged outside the dip tank and by means of which the acid formed during the coating operation can be removed from the coating liquid, such that the acid content in the coating liquid remains within an admissible range.
2. Description of Related Art
In prior art installations for the cataphoretic dip coating of articles, the electrodialysis apparatuses, which ensured a substantially constant pH or conductance value of the coating liquid, were fitted inside the dip tank itself. The corresponding dialysis cells consisted of a housing, which bore an ion-exchange membrane which allowed anions to pass through, but was impermeable to cations and coating solids. The ion-exchange membrane here surrounded the anodes which were responsible for the cataphoretic coating operation. This design inevitably resulted in the anodes' being linear structures. However, since the articles to be coated themselves have an irregularly shaped outline, i.e. have for example curved surfaces, in the stated known installations non-uniform electrical fields were obtained between the anode and the articles to be coated, with the consequence that the thickness of the coating over all the areas of the article to be coated could be kept constant only with difficulty. In order to avoid excessively high local current densities, coating speed moreover had to be limited.
Cataphoretically operated installations with the electrodialysis apparatus fitted in the dip tank had certain further disadvantages which have resulted in the electrodialysis apparatus being removed from the dip tank in more recent known installations and arranged outside said tank with its own voltage source. Examples of such installations of the above-stated type are described in DE 32 43 770 A1, JP 55-00 6452 A and JP 55-05 0493 A. However, relocating the electrodialysis apparatus outside the dip tank in these latter-stated installations did not change the anodes from being linear structures. These installations accordingly also exhibit the above-described disadvantages.